1. Field of the Invention
The present invention relates to an electronic throttle control device for controlling an engine for a vehicle, for example, and particularly to an improvement of an electronic throttle control device for an engine with which the detection precision of a throttle opening degree can be enhanced by using an inexpensive AD converting means having relative low resolution.
2. Description of the Related Art
An electronic throttle control device for an engine electronically controls a throttle valve for adjusting an suction air amount of the engine, and is generally equipped with a throttle valve, a throttle opening degree detecting unit and a throttle valve control circuit. The throttle opening degree detecting unit generates an analog opening degree detection signal whose magnitude is proportional to the opening degree of the throttle valve. The throttle valve control circuit receives a throttle opening degree target signal and the analog opening degree detection signal, and controls the opening degree of the throttle valve so that the analog opening degree detection signal corresponds to the opening target signal.
The throttle valve control circuit digitally controls the opening degree of the throttle valve by using a microcomputer. In connection with this digital control, the analog opening degree detection signal from the throttle opening degree detecting unit is converted to a digital opening detection signal by using an AD converting means. The AD converting means is constructed by using a microcomputer, however, in order to reduce the cost of the microcomputer, an inexpensive AD converting means having low resolution is desirable.
JP-A-2003-28001 discloses a prior art which uses an AD converting means having low resolution and also can enhance the detection precision of the throttle opening degree. The AD converting means disclosed in this prior art has a level converting circuit in which plural resistors are connected to an analog input portion in series, and also has an adding means at a digital output portion. The level converting circuit generates plural analog signals different in level on the basis of the analog opening degree detection signal. The AD converting unit converts the plural analog signals to corresponding digital signals, and generates plural digital outputs. These plural digital outputs are added with one another in the adding unit.
The plural analog signals generated by the level converting circuit are called as offset-attached voltages. The plural offset-attached voltages are different from one another in analog level, and the analog levels of all the offset-attached voltages are varied in connection with variation of the analog opening degree detection signal. These plural offset-attached voltages are converted to digital outputs by the AD converting means, and added in the adding means, so that the AD converting means has higher conversion precision surpassing there solution thereof. By using the AD converting means as described above, the throttle opening degree can be detected with higher precision while using a more inexpensive AD converting means.
However, in the above prior art, it is necessary to convert each of the plural offset-attached voltages to the corresponding digital output at a predetermined sampling timing, and thus it takes a long time to carry out the AD conversion processing. In general, in the case of a vehicle, a microcomputer for controlling an engine is commonly used among plural control operations to the engine, and thus the increase of the AD conversion processing time for the electronic throttle control may be an obstacle to the other control operations of the engine. In order to avoid this disadvantage, it is necessary to use an AD converting means having a short processing time for the electronic throttle control, and thus an expensive microcomputer having a high processing speed must be adopted, so that the prior art described above cannot be actively applied and thus the cost is increased.
For example, when a microcomputer for carrying out fuel injection control for an engine and ignition control of the engine is commonly used to electronic throttle control, it is necessary detect the crank angle of the engine with high resolution in order to control the combustion state of the engine with high precision. The crank angle signal (for example, a pulse signal is generated every crank angle of 6 degrees) is input to interruptive processing of the microcomputer at high speed in proportion to the rotational speed of the engine, so that the processing load of the microcomputer is increased in proportion to the rotational speed of the engine. Therefore, in the case of a microcomputer having a low processing speed, it is concerned that reset occurs due to deficient processing time, and in order to avoid this, it is necessary to use an expensive microcomputer having a high processing speed, resulting in increase of the cost.